Caption: Ammonia production by the Haber process. This makes ammonia (NH3) from nitrogen (N2) and hydrogen (H2) gas. Hydrogen (red) and nitrogen (blue) are first cleaned (lower left), mixed, and then compressed to 200 atmospheres by the grey compressor. The mixture cycles through the reaction tower (down right), which contains horizontal trays carrying an iron catalyst and is at a temperature of 450 degrees Celsius. The ammonia gas (pink) that forms in the tower passes over a cooling loop (pale blue) at the base of the reaction tower, which condenses the ammonia into a liquid (orange), which is piped off at lower right. Ammonia is widely used in the chemical industry to produce other chemicals, particularly fertilisers, and the process seen here is an important method of fixing nitrogen from the atmosphere.